Holograms and other types of diffraction gratings comprising reflective metal surface on a relief-patterned substrate are commonly used for decorative packaging, artistic images and security devices, e.g. on credit cards, currency and other official documents. Such holograms can be prepared by a variety of methods such as embossing deformable metal foil laminates with a holographic image stamp. Due to the resilience of laminate components, such embossed holograms tend to be of poor quality especially when subjected to elevated temperatures.
An alternative method is disclosed by D'Amato et al. in European Patent Publication 0 338 378 where a holographic polymeric substrate is formed by casting and curing a polymer precursor in contact with a holographic relief-patterned mold to form a polymeric substrate with a holographic relief-patterned surface; a metal reflective surface is then deposited on the relief-patterned surface, e.g. by vapor deposition techniques. The resulting relief-patterned metal surface serves to reflect incident light into a reconstructed image of the hologram. A disadvantage of such metal deposition is the requirement to conduct such metal deposition in a vacuum environment and to mask areas where metal deposition is not desired especially if the hologram is mounted on a document.
An object of this invention is to provide such reflective metal holograms which can be prepared by metal deposition techniques that do not require vacuum environments or masking. One method for depositing metal onto polymeric substrates is electroless deposition. However, electroless deposition of metal has some disadvantages which do not commend it to such hologram preparation. For instance, many techniques for electroless deposition of metal onto plastics require etching to achieve a effective level of adhesion; such etching, e.g. with strong acids or solvents, can destroy the holographic relief pattern of the polymeric substrate surface. Moreover, electroless deposition techniques often utilize catalytic materials of a size on the order of magnitude of the holographic relief pattern; deposition of dispersed particulate catalytic materials can distort the reproduction of the holographic relief pattern in the metal coating.